The present invention relates to an improved fabrication method for holders for materials used in measuring instruments such as calorimeters, and it is particularly useful in a differential scanning calorimeter.
The present invention particularly relates to a material holder used in a differential scanning calorimeter of the type disclosed and claimed in U.S. Pat. No. 3,732,722 for a "MATERIAL HOLDER" issued May 15, 1973 and invented by Norem et al and assigned to the Perkin Elmer Corporation of Norwalk, Conn. A differential scanning calorimeter is a thermal analytical instrument which operates on the principle that thermal energy is absorbed or evolved during physical or chemical changes in a material which is being analyzed. The differential scanning calorimeter measures the differential energy changes that occur in a sample material as compared to a reference material, during such physical or chemical changes. The sample material and a thermally inert reference material are placed in separate material holders in the same thermal environment and their temperatures are measured during the analysis process.
The material holder, as disclosed in the aforementioned patent, consists of a cylindrical cup having three bottom partitions forming two cavities within the bottom portion of the cup, one of the cavities containing a heating winding, and the other cavity containing a heat-sensing winding. A supporting center post is attached to the bottom of the lowermost partition as a support for the material holder.
In order to produce the material holder, a series of substantially identical discs are punched from sheet stock with a punch and die set to form the three partitions. These discs are successively press fitted into a cylindrical housing and electron beam welded at the disc edges to the cylindrical housing. The heater and sensing windings are sandwiched between the successive disc partitions.
Serious problems have been encountered in the above-described assembly and fabrication method in that it has been found to be extremely difficult to maintain tolerances on the disc diameters and the inside diameters of the cylindrical housings with sufficient precision to avoid very high rejection rates in the assembled material cups. The success of the instrument resides in maintaining extreme precision. The precision in measurement is so important that a noble metal such as platinum, paladium, gold, and their alloys are preferably used as the material of the material holder. Preferably, the metal of which the holder is formed is an alloy of 80% platinum and 20% iridium. The noble metals avoid the problems of oxide formation.
The high precision also requires extremely narrow tolerances in the assembled dimensions of the material holders. Thus, in a material holder which is in the order of one-third of an inch in diameter (actually 0.360 inches in one preferred embodiment), the various partitions must be perfectly parallel with one another with a variation out of parallelism not to exceed 0.001 inch. Also, the various partitions must not be distorted in shape, the compartments containing the windings must be absolutely consistent and uniform in size, within the tolerance of plus or minus seven ten thousandths of an inch, and there must be very good conductivity between the edges of the partition discs and the cylindrical housing. In order to accomplish these purposes, the partitions must be press fitted into position within the cylindrical housing with a tight enough fit to precisely maintain the position of each disc during handling after assembly and before electron beam winding. Furthermore, the press fit must be sufficiently tight to promote the production of a good sound electron beam weld between the disc partition and the cylindrical housing in order to provide a consistently high thermal conductivity through the joint formed thereby. On the other hand, the disc must not be press fitted with so much of an interference fit that it results in distortion of the disc.
One of the biggest problems in achieving the satisfactory press fit of the disc partitions into the cylindrical housing apparently arises because of variations in the diameters of the discs produced by the punch and die set. For a particular batch run of approximately 900 discs between sharpenings of the punch and die set, it has been discovered that the disc diameters may vary in a typical range of about 3/10,000ths of an inch, or more. These variations in disc diameter are believed to be associated with the wear pattern of the punch and die, and also, possibly, these variations may be related to variations in the toughness and thickness of the sheet metal from which the discs are punched.
Accordingly, it is one object of the present invention to provide an improved fabrication method which avoids the problems arising from variations in the disc diameters as produced by the punch and die.
Another problem in providing the correct press fit arises particularly in connection with the bottommost disc partition to which the support post is welded. It has been discovered that the support post welding causes the disc to which it is welded to shrink somewhat in diameter, that shrinkage being in the order of 4 to 5 ten thousandths of an inch.
Accordingly, it is another object of the invention to provide an improved fabrication method which avoids the problem associated with the shrinkage in the diameter of the bottommst disc partition occasioned by the welding of the support post thereto.
Another major problem in producing a satisfactory press fit of the disc partitions has been found to result from the fact that the electron beam welding of the first (uppermost) partition into the cylindrical housing causes the diameter of that housing to shrink slightly, so as to increase the tightness of the cylinder around the edges of subsequently assembled disc partitions. Similarly, the electron beam welding of the second disc partition causes still a further shrinking of the cylindrical housing for the assembly of the third disc partition.
Accordingly, it is another object of the present invention to provide an improved fabrication method which avoids the consequences of the problem of the shrinkage of the cylindrical housing resulting from the electron beam welding of earlier assembled disc partitions in order to improve the press fit of subsequently assembled disc partitions.
Further objects and advantages of the invention will appear from the following description and the accompanying drawings.